/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.api.common.functions;

import org.apache.flink.annotation.Public;

import java.io.Serializable;

/**
 * Generic interface used for combine functions ("combiners"). Combiners act as auxiliaries to a
 * {@link GroupReduceFunction} and "pre-reduce" the data. The combine functions typically do not see
 * the entire group of elements, but only a sub-group.
 *
 * <p>Combine functions are frequently helpful in increasing the program efficiency, because they
 * allow the system to reduce the data volume earlier, before the entire groups have been collected.
 *
 * <p>This special variant of the combine function reduces the group of elements into a single
 * element. A variant that can return multiple values per group is defined in {@link
 * GroupCombineFunction}.
 *
 * @param <IN> The data type processed by the combine function.
 * @param <OUT> The data type emitted by the combine function.
 */
@Public
@FunctionalInterface
public interface CombineFunction<IN, OUT> extends Function, Serializable {

    /**
     * The combine method, called (potentially multiple timed) with subgroups of elements.
     *
     * @param values The elements to be combined.
     * @return The single resulting value.
     * @throws Exception The function may throw Exceptions, which will cause the program to cancel,
     *     and may trigger the recovery logic.
     */
    OUT combine(Iterable<IN> values) throws Exception;
}
